Comparative Quantitative Prevalence of Mycobacteria and

The Chattanooga Creek Superfund site is heavily contaminated with metals, pesticides, and coal tar with sediments exhibiting high concentrations of po...
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Environ. Sci. Technol. 2007, 41, 5426-5432

Comparative Quantitative Prevalence of Mycobacteria and Functionally Abundant nidA, nahAc, and nagAc Dioxygenase Genes in Coal Tar Contaminated Sediments J E N N I F E R M . D E B R U Y N , †,§ CHRISTOPHER S. CHEWNING,§ AND G A R Y S . S A Y L E R * ,†,‡,§ Department of Ecology and Evolutionary Biology, Department of Microbiology, and Center for Environmental Biotechnology, University of Tennessee, Knoxville, TN

The Chattanooga Creek Superfund site is heavily contaminated with metals, pesticides, and coal tar with sediments exhibiting high concentrations of polycyclic aromatic hydrocarbons (PAHs). High molecular weight PAHs are of concern because of their toxicity and recalcitrance in the environment; as such, there is great interest in microbes, such as fast-growing Mycobacterium spp., capable of degradation of these compounds. Real-time quantitative PCR assays were developed targeting multiple dioxygenase genes to assess the ecology and functional diversity of PAHdegrading communities. These assays target the Mycobacterium nidA, β-proteobacteria nagAc, and γ-proteobacteria nahAc with the specific goal of testing the hypothesis that Mycobacteria catabolic genes are enriched and may be functionally associated with high molecular weight PAH biodegradation in Chattanooga Creek. Dioxygenase gene abundances were quantitatively compared to naphthalene and pyrene mineralization, and temporal and spatial PAH concentrations. nidA abundances ranged from 5.69 × 104 to 4.92 × 106 copies per gram sediment; nagAc from 2.42 × 103 to 1.21 × 107, and nahAc from below detection to 4.01 × 106 copies per gram sediment. There was a significantly greater abundance of nidA and nagAc at sites with the greatest concentrations of PAHs. In addition, nidA and nagAc were significantly positively correlated (r ) 0.76), indicating a coexistence of organisms carrying these genes. A positive relationship was also observed between nidA and nagAc and pyrene mineralization indicating that these genes serve as biomarkers for pyrene degradation. A 16S rDNA clone library of fastgrowing Mycobacteria indicated that the population is very diverse and likely plays an important role in attenuation of high molecular weight PAHs from Chattanooga Creek.

Introduction Microbial biodegradation is the primary degradation fate pathway for polycyclic aromatic hydrocarbons (PAHs) in soils * Corresponding author phone: 865-974-8080; fax: 865-974-8086; e-mail: [email protected]. † Department of Ecology and Evolutionary Biology. ‡ Department of Microbiology. § Center for Environmental Biotechnology. 5426

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and sediments. PAHs are toxic, mutagenic, carcinogenic, and have been associated with inflammatory vascular responses (1). Many studies focus on aerobic dioxygenase-mediated degradation of lower molecular weight PAHs, such as naphthalene and phenanthrene, which are more soluble, and thus more easily biodegraded. More recently, Mycobacterium capable of using high molecular weight (HMW) PAHs (e.g., pyrene and fluoranthene) as a sole carbon source have been isolated (2, 3). These PAH-degrading Mycobacteria are fast-growers, a distinct biochemical and phylogenetic taxonomic division within the genus (4). Their ability to degrade HMW PAHs is partially due to the highly hydrophobic mycobacterial cell wall (5, 6) suggesting a selective advantage in obtaining PAHs from the environment. Mycobacterium spp. have been isolated and characterized from PAHcontaminated environments (2, 7-12) and in vivo biochemical pathways of pyrene degradation have been identified (13, 14). The genes encoding the large (R) and small (β) subunits (nidA and nidB) of the Rieske-type dioxygenase responsible for initial pyrene dihydroxylation have been sequenced in several strains (15). Recent advances in molecular microbial ecology have allowed for broader analysis of biodegradative organisms in the environment. Molecular methods targeting 16S ribosomal (rDNA) genes have been effective in resolving phylogenetic diversity in PAH-degrading Mycobacterium populations (16, 17). However, 16S methods make no inference as to the function of these communities. Identification and quantification of catabolic genes is one way to functionally characterize microbial communities. In PAH-contaminated sites, previous studies have revealed an enrichment of PAHcatabolism genes, specifically naphthalene dioxygenases. For example, γ-proteobacteria naphthalene dioxygenase genes (nahAc) have been related to naphthalene concentrations and/or naphthalene mineralization rates in the environment (18-20). Dionisi et al. (21) found a positive correlation between β-proteobacteria naphthalene dioxygenase genes (nagAc) and naphthalene concentrations in Chattanooga Creek sediments. Quantification of catabolic genes can provide insight into the role of particular groups of bacteria in natural attenuation of PAHs. The Chattanooga Creek Superfund Site (Chattanooga, TN) was placed on the National Priorities list in 1995 due to mixed priority pollutants and heavy coal tar contamination; predominantly from a nearby coking coal plant (22). This contamination has resulted in high concentrations of PAHs in the sediments (21). HMW PAHs are of particular concern because of their toxicity, carcinogenicity, and recalcitrance due to low solubility, high thermodynamic stability, and slow microbial degradation (23). Tar-rich sediments were excavated from a section of Chattanooga Creek in 1998; however, despite these remediation efforts PAH concentrations have remained high (21). In addition, measurable concentrations of PAHs on the floodplain indicate that there is likely frequent redeposition of contaminated sediments during flood events (22, 24). The unsuccessful physical remediation and dynamic nature of this system indicates that further understanding of natural attenuation of PAHs by microorganisms is still needed. To test the hypothesis that fast-growing Mycobacterium nidA genes are enriched in contaminated sediments and are functionally associated with HMW PAH degradation in Chattanooga Creek, a quantitative real-time PCR assay was designed to quantify pyrene dioxygenase gene (nidA) abundance in indigenous microbial communities. While primers have been designed to detect this gene in isolates (7) and in 10.1021/es070406c CCC: $37.00

 2007 American Chemical Society Published on Web 06/27/2007

FIGURE 1. Map of sampling locations on Chattanooga Creek. The Tennessee River is pictured in the top left-hand corner; the Tennessee-Georgia border is along the bottom. Zone I is upstream of point sources of contamination (sources include the Tennessee Products coking coal carbonization facility, in operation from 1918 to 1987). Zone II sediments were excavated in 1998; Zone III is currently being excavated (2006-2007).

soil (25), this study is the first to quantitatively measure copies of nidA and relate catabolic gene frequency to other parameters (such as mineralization rates and PAH concentrations) to evaluate this target as a biomarker of biodegradation. In addition, a Mycobacterial 16S clone library has been developed to assess the diversity of Mycobacteria indigenous to this contaminated site. 16S rDNA genes have been a target for other studies of fast-growing Mycobacterium (16, 17); this study presents the first Mycobacterium clone library from the Chattanooga Creek Superfund Site.

Materials and Methods Sampling. Sediment samples were collected from Chattanooga Creek, a shallow, lotic system in October 2004, and April, July, and October 2005. Samples were collected from a site upstream of the Superfund Site (W), from a section that was excavated in 1998 (DO), and from an unremediated, highly contaminated site (CF) (Figure 1) (corresponding to sites 1, 4, and 6, respectively, from Dionisi et al. (21)). The control site (W) is upstream of the highly contaminated area, however, still exhibits low levels of contamination due to heavy industry and traffic in the area. At all sites, surface sediments were fine sand, with no visible coal tar (unlike deeper sediments). Triplicate sediment samples were taken from the top 5-10 cm (aerobic zone), stored in Whirlpak bags, and either frozen immediately on dry ice for DNA extraction or kept at ambient temperature for mineralization assays. PAH Quantification. PAHs were extracted according to EPA method 3546 using the Microwave Automated Reaction System from CEM (Matthews, NC). Briefly, 10 g sediment were combined with Base/Neutral Surrogate Standard (UltraScientific, North Kingstown, RI) and 5 g Na2SO4. Samples were ground to 80 °C for at least 48 h. Mineralization Assays. To determine active PAH mineralization rates, three vials (and one killed control) were used for each sample: 2 g sediment was slurried with 1 mL dH2O in a 40 mL EPA vial with a septum. As a positive control, a 1 mL overnight culture of Mycobacterium flavescens (ATCC 700033) (pyrene) or Pseudomonas fluorescens HK44 (naphthalene) were used in place of sediment. One mL 2N H2SO4 was added to each killed control; an 8 mL vial with 0.5 mL 0.5 N NaOH was placed in each to serve as a CO2 trap. Either 400 000 dpm naphthalene-1-14C (31.3 mCi/mmol, Sigma, St. Louis, MO) or 200 000 dpm pyrene-4,5,9,10-14C (55 mCi/ mmol, Sigma, St. Louis, MO) dissolved in acetone were added to each. Vials were dark-incubated at room temperature with shaking for 40 h. The NaOH was mixed with water and ReadySafe liquid scintillation cocktail (Beckman Coulter, Fullerton, CA) and assayed on a Packard TriCarb 2900TR liquid scintillation analyzer (Perkin-Elmer, Downers Grove, IL). 14C efficiencies (0-156 keV) ranged from 94.37 to 96.18. DNA Extraction and Real-Time PCR. Total DNA was purified in triplicate from sediment samples using the FastDNA SPIN kit for soil (Qbiogene, Morgan Irvine, CA) with minor modifications as described previously (21). Primer and probe sets for quantifying dioxygenase genes targeted genes encoding the large (R) dioxygenase subunit (nidA, nagAc, and nahAc). For the nidA TaqMan probe quantitative PCR assay, primers and probes target conserved regions determined from a multiple alignment of nidA from several PAH-degrading Mycobacterium: M. flavescens (AF548343), M. fredrickbergense (AF548345), M. gilvum (AF548347), and M. sp. strains PYR-1 (AF249301), 6PYR (AJ494745), JLS (AY330098), KMS (AY330100), MCS (AY33010), and MHP-1 (AB179737). The TaqMan probe (5′-FAMTCCTACCCGTCGCCGGTACA-BHQ1) contained several base pair differences to closely related Rhodococcus sp. NCIMB narA (AF082663) gene. Forward and reverse primer sequences were 5′-TTCCCGAGTACGAGGGATAC and 5′-TCACGTTGATGAACGACAAA, respectively. The real-time PCR reactions used QuantiTect Probe PCR Master Mix (Qiagen, Valencia, CA) and were performed on an MJ Opticon thermocycler (Bio-Rad, Hercules, CA) using the following protocol: 50 °C for 2 min, 95 °C for 15 min, then 40 cycles of denaturing at 94 °C for 15 s and annealing at 56 °C for 30 s. nidA from Mycobacterium flavescens (ATCC 700033) cloned into TOPOTA-PCR4 vector (Invitrogen, Carlsbad, CA) was used to create an internal eight-point standard curve ranging from 101 to 108 gene copies per reaction. To ensure specificity of the assay, PCR products were cloned. Twenty clones were sequenced using M13 reverse primer on an ABI3100 genetic analyzer (Applied Biosystems, Foster City, CA) at the Molecular Biology Resource Facility, University of Tennessee (Knoxville, TN). A TaqMan probe quantitative PCR assay targeting conserved regions of nahAc genes encoding naphthalene dioxygenase in γ-proteobacteria was also developed. Eighteen Pseudomonas were used to design degenerate primers and probe, including, P. stutzeri (AF306425), P. balearica (AF306428), P. fluorescens (AY125981), P. putida strains (AF306430, AF306441, AF306439), and related Pseudomonas spp. Forward and reverse primer sequences were 5′-CAGAGCGTYCCRTTYGAAAA and 5′-TCGAAGCAACCRTARATGAA, respectively. The TaqMan probe sequence was 5′-FAMTGGGGTTGAAAGAAGTCGCTCG-BHQ1. Real-time PCR assays were performed as described above, using an annealing temperature of 52 °C, and cloned nahAc from Pseudomonas putida G7 as the internal standard. Copies of nagAc-like genes in β-proteobacteria as well as universal 16S rDNA genes were quantified as previously described (21, 26). VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Quantities of 16S (black), nidA (white), nagAc (hatched), and nahAc (gray) at the three sites for each sampling date. Bars are means of triplicate PCR on triplicate DNA extractions from triplicate sediment samples taken at each site. Error bars are ( standard deviation (n ) 27). Statistical comparisons of gene abundance were done in NCSS (Number Cruncher Statistical Systems, Kaysville, UT) using a nonparametric one-way ANOVA on the ranked data, and Kruskal-Wallis Multiple Comparison Z test with Bonferroni adjustment. Comparisons were made between sites (W, DO, CF) and between sampling dates; ANOVA results with p < 0.01 were considered statistically significant. Mycobacterium 16S Clone Library. Primers designed by Leys et al. (16) to target an approximately 530 bp region of the 16S ribosomal gene in fast-growing Mycobacterium (corresponding to positions 66-600 in Escherichia coli) were used to create a clone library using DNA extracted from site DO. A touchdown PCR protocol was used (95 °C for 10 min, then 20 cycles of denaturing at 95 °C for 30 s, annealing at 55 °C - 1 °C/cycle for 30 s and extension at 72 °C for 30 s, followed by 10 cycles with an annealing step at 45 °C, and a final extension of 72 °C for 10 min). PCR products were cloned into a TOPO-TA-PCR4 vector (Invitrogen, Carlsbad, CA). Sequencing was done as described above. Alignments and phylogenetic trees were done using MEGA (Molecular Evolutionary Genetics Analysis) software version 3.1 (http:// www.megasoftware.net/). Sequences were deposited in the NCBI GenBank, accession numbers EF438310-EF438383.

Results Optimization and Specificity of the nidA Real-Time PCR Assay. Initially, PCR was performed with a gradient of annealing temperatures to determine optimal temperature (59 °C); and different concentrations of primers and probe were combined to determine the optimal combination of 300 nM probe and 600 nM each primer per reaction (data not shown). For both cloned nidA and total sediment DNA extracted, the assay yielded an expected linear response in threshold cycle between 1 ng and 100 ng per reaction, indicating no PCR inhibition (Figure S1). To ensure specificity of the nidA real-time PCR assay, 20 cloned products were sequenced, all showing 99-100% similarity with PAH-degrading Mycobacterium sp. nidA genes 5428

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(data not shown). This is expected as this assay targets a conserved region of nidA, and this gene is highly homologous between species (7). Quantification of nidA, nagAc, and nahA in Sediments of Chattanooga Creek. 16S rDNA genes were quantified in all samples as a proxy for bacterial biomass (26). In October 2004 and July and October 2005, abundance of 16S rDNA copies (biomass) ranged from 2.64 × 108 to 3.70 × 109 copies per gram dry weight sediment (Figure 2). In April, there was approximately an order of magnitude more biomass, ranging from 1.72 × 109 to 9.67 × 109 copies per gram sediment. A nonparametric one-way ANOVA indicated that there was no significant biomass difference between the three sampling sites, confirming that differences in dioxygenase gene abundance between sites were not simply due to differences in biomass. Between sampling sites there was a significant difference in quantities of nidA and nagAc: both exhibited significantly lower copies at site W. In addition, there were no significant differences in quantities of nidA or nagAc between dates at site W. These observations are consistent with the fact that site W has not experienced the historical contamination and typically exhibits lower concentrations of PAHs than site DO and CF. nidA copies were not significantly different between DO and CF, and nagAc copies were highest at DO. These patterns are also observed in Figure S2, which shows quantities of each gene by sampling event. nahAc abundances were quite variable: Quantities varied about 3 orders of magnitude between October 2004 and October 2005, and in some cases, quantities were below the detection limit of the assay (>100 copies/g sediment). This observation is in contrast to nidA and nagAc, which were detected in all samples. Across sampling dates there was not as clear of a pattern. There were significant differences in copies of nidAc: greatest abundances were observed in October 2004 and April 2005, followed by July and October 2005. No significant differences existed in nagAc quantities; however, there was a significantly

TABLE 1. Spearman Rank Correlation Coefficients (rs) Matrix NapMin PyrMin NapMin PyrMin 16S/g 16S/µg nid/g nid/µg nid/16S nag/g nag/µg nag/16S nah/g nah/ug nah/16S naph phen pyr benz-a chry benz-gh

1.00 0.78 0.14 -0.15 0.23 0.02 0.33 0.67 0.36 0.60 -0.31 -0.37 -0.21 0.28 0.82 0.75 0.63 0.73 0.80

1.00 0.35 0.15 0.49 0.35 0.28 0.81 0.57 0.55 -0.01 -0.09 -0.09 0.58 0.58 0.73 0.78 0.62 0.78

16S /g

1.00 0.85 0.36 0.51 -0.33 0.52 0.68 0.41 -0.16 -0.24 -0.58 -0.37 -0.38 -0.48 -0.32 -0.52 -0.50

16S /µg

nid /g

nid nid nag /µg /16S g

nag /µg

nag /16S

nah /g

nah nah /µg /16S naph phen pyr benz-a chry

1.00 0.21 1.00 0.43 0.93 1.00 -0.48 0.66 0.45 1.00 0.34 0.76 0.69 0.45 1.00 0.66 0.58 0.69 0.10 0.78 1.00 0.27 0.48 0.52 0.34 0.80 0.85 1.00 -0.09 0.21 0.27 0.26 0.06 -0.19 -0.18 1.00 -0.09 0.19 0.25 0.29 0.01 -0.20 -0.20 0.98 1.00 -0.45 0.07 0.06 0.44 -0.09 -0.32 -0.16 0.86 0.89 1.00 -0.31 0.23 0.16 0.34 0.45 -0.06 0.03 0.65 0.51 0.70 -0.53 -0.17 -0.45 0.22 0.25 -0.20 0.18 -0.01 -0.14 0.26 -0.53 0.30 0.05 0.60 0.57 0.08 0.38 0.18 0.08 0.54 -0.32 0.27 0.17 0.47 0.63 0.23 0.45 0.33 0.22 0.60 -0.63 0.15 -0.17 0.47 0.37 -0.12 0.22 0.04 -0.09 0.39 -0.52 0.23 -0.03 0.55 0.53 0.07 0.35 0.14 0.06 0.53

1.00 0.49 0.76 0.85 0.66 0.73

1.00 0.80 0.68 0.88 0.83

1.00 0.93 1.00 0.95 0.82 0.98 0.92

1.00 0.93

a NapMin and PyrMin are the percentage of added naphthalene and pyrene mineralized in 40 h. 16S, nid, nag and nah are abundance of 16S rDNA, nidA, nagAc, and nahAc genes, respectively, normalized to g dry weight sediment (/g), µg DNA extracted (/µg), or to 16S rDNA abundances (/16S). Naph, phen, pyr, benz-a, chry, and benz-gh are concentrations of naphthalene, phenanthrene, pyrene, benzo[a]pyrene, chrysene, and benzo[g,h,i]perylene, respectively, measured in mg PAH per kg dry weight sediment. Significant correlations are italic (n ) 12, rs > 0.591, R ) 0.05).

greater abundance of nahAc in October 2004 compared to other sampling dates. PAH Concentrations. Sixteen priority PAHs were measured at each site and each sample date (Table S1). At site W, naphthalene was not detected in April or July 2005; in October 2004, we measured 0.07 mg naphthalene/kg sediment (dry weight). Average concentrations of pyrene ranged from 0.30 to 0.44 mg/kg and concentrations of benzo[a] pyrene up to 0.33 mg/kg. At site DO, average naphthalene concentrations ranged from undetectable to 0.07 mg/kg; pyrene from 0.69 to 9.75 mg/kg; and benzo[a]pyrene from 1.77 to 8.57 mg/kg. Site CF had the highest PAH concentrations with naphthalene ranging from 0.34 to 2.63 mg/kg; pyrene ranging from 9.16 to 22.43 mg/kg; and benzo[a]pyrene from 15.25 to 36.34 mg/kg. Concentrations of PAHs were mostly consistent with previously measured concentrations at this site (21), with the exception of naphthalene which was lower in this study. Concentrations of PAHs with three or more rings were strongly correlated (Table 1). PAH Mineralization. Naphthalene mineralization was observed in all samples over 40 h. At site W, 1.2-35.6% of added naphthalene was mineralized; at site DO, 7.1-99.2% was mineralized, and at site CF, 41.8-80.7%. Pyrene mineralization was very low at site W, ranging from 0.1 to 0.8% of pyrene added, but higher at sites DO (11.1-25.8%) and CF (16.6-26.9%). The amount of pyrene mineralized corresponds with observations made by Cheung and Kinkle (17): inoculated Mycobacterium mineralized less than 40% of added pyrene, even after 100 days. Figure 3 shows the curvilinear relationship of measured mineralization to PAH concentrations in Chattanooga Creek sediments: biodegradative capabilities are positively related to PAH concentration only at low PAH concentrations Correlations Among Catabolic Genotypes and Mineralization Potential. To determine the relationship between dioxygenase gene quantities and other variables which may be related to the microbial community’s degradative potential, a correlation matrix of Spearman rank correlation coefficients (rs) was developed (Table 1). As expected, a significant correlation was observed between naphthalene and pyrene mineralization (rs ) 0.783), and mineralization is positively correlated to concentrations of most individual PAHs (Table 1). A significant correlation also exists between

FIGURE 3. Napthalene (solid circles) and pyrene (open circles) mineralization (40 h incubation) in relation to measured concentrations of naphthalene (A) and pyrene (B). Error bars represent the standard deviation of triplicate mineralization assays on triplicate sediment samples from each site. VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Functional relationship between PAH-dioxygenase gene abundances (b nidA, O nagAc,1 nahAc) and in vitro pyrene mineralized. Best fit lines are described by logarithmic nonlinear regression (% pyrene mineralized ) y0 + a ln (gene copies /g)): for nidA (% pyrene mineralized ) -41.47 + 4.12 ln (nidA/g)), r2 ) 0.74 (long dashes); for nagAc (% pyrene mineralized ) -27.55 + 3.46 ln (nagAc/g)), r2 ) 0.93 (short dashes). nidA and nagAc copies per gram dry weight sediment (rs ) 0.755, p > 0.001) (Figure S3). Weak but significant correlations between dioxygenase genes and PAHs include nidA (normalized per biomass) and pyrene (rs ) 0.600), nagAc and benzo[a]pyrene (rs ) 0.633), nahAc (normalized per biomass) and benzo[a]pyrene (rs ) 0.600) and nahAc and naphthalene (rs ) 0.647). One goal of this study was to assess dioxygenase genes as markers for active biodegradation in this system. The relationship between catabolic gene quantities and pyrene mineralization is shown in Figure 4. Both nagAc and nidA show positive relationships to pyrene mineralization. Best fit lines for each data set are described by logarithmic nonlinear regression (% pyrene mineralized ) y0 + a ln (gene copies /g)): for nidA (% pyrene mineralized ) -41.47 + 4.12 ln (nidA/g)), r2 ) 0.74; nagAc (% pyrene mineralized ) -27.55 + 3.46 ln (nagAc/g)), r2 ) 0.93. Diversity of Mycobacteria. Detection of nidA in Chattanooga Creek sediments implicates the presence of fastgrowing, potentially PAH-degrading Mycobacteria. To further investigate Mycbacterial diversity, fast-growing Mycobacterium 16S rDNA genes were cloned. Seventy-four clones were successfully sequenced, all showing high sequence similarity (96-100%) to other published fast-growing Mycobacterium. Few clones were duplicate sequences: instead the majority was unique, indicating a high diversity of Mycobacterium in this system. A neighbor-joining tree using the Kimura 2-parameter distance model (27) shows the phylogenetic relationship of these sequences (Figure 5). As expected, most clustered apart from M. ulcerans, a slow-growing Mycobacterium sp. Most sequences fall into the same cluster as M. vanbaalenii, M. austroafricanum, and M. flavescens, species that have all been characterized as high molecular weight PAH degraders (12), suggesting that these clones represent other PAH-degrading Mycobacterium. Two clones were most closely related (97-98%) to opportunistic pathogens such as Mycobacterium mucogenicum (AJ627393).

Discussion Catabolic genes represent the degradative potential of a bacterial community, and are indicative of the community’s 5430

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response to bioavailable substrate. It is hypothesized that catabolic genes can serve as markers of actual function: in the case of PAH-degrading communities, strong positive correlations have previously been found between nahAc gene copies and transcripts (18, 19) indicating that the presence of genes in a community is related to their activity. In this study we quantified three different PAH dioxygenase genotypes to better understand the relationship to each other within a community, as well as to actual function (PAH mineralization) of these communities. While many studies have used molecular tools such as PCR to detect single catabolic genes during biodegradation (21, 28), multiple targets give a more comprehensive picture of community-level responses, and can provide insight into relationships between populations carrying the genotypes. Multiple catabolic gene targets have been successfully used to establish microbial biodegradative potential in an aquifer (29) and within a BTEX plume (30), and have also provided insight into microbial communities exposed to PAHs (20, 31, 32). In this study, naphthalene dioxygenase genes nagAc and nahAc (from β- and γ-proteobacteria, respectively), as well as the pyrene dioxygenase gene nidA (from Mycobacterium) were not only identified, but also quantified. Establishment of dioxygenase genes within these communities combined with measured mineralization indicates that aerobic PAHdegradation is occurring; this observation is consistent with the creek’s frequent aeration due to flood disturbances and its lotic nature. Abundances of nidA and nagAc were lower at site W, which has the lowest levels of contamination and serves as the control site for this study. Ralstonia-like nagAc abundances measured in our study (103-107 copies/g sediment) correspond with those previously measured in Chattanooga Creek by Dionisi et al. (21) (105-107 copies/g sediment), however, do not show the same strength of correlation with naphthalene concentrations (r ) 0.45 in this study compared to r ) 0.77 by Dionisi et al.). This is likely a result of lower naphthalene concentrations (sometimes below detection) measured during our study. A significant positive correlation was observed between copies of nidA and nagAc in Chattanooga Creek sediments

be inactive. While many cultured PAH-degrading organisms have nahAc-like genes, these results suggest that γ-proteobacteria carrying nahAc may not play a major role in Chattanooga Creek sediment communities in terms of PAH degradation. In microcosms amended with low molecular weight PAHs, Laurie and Lloyd-Jones (31) observed an increase in phnAc genes (from Burkholderia), but did not detect nahAc, suggesting that this genotype may not be ecologically relevant in all environments. This is also supported by other studies indicating that nah genes alone underestimate total biodegradative capacity (33). In addition, populations containing nahAc have been found to be transient with respect to contaminant concentrations (30), possibly explaining the variability in nahAc abundance seen in this study. Mineralization of pyrene was positively correlated to abundance of nagAc and nidA. As this is, to our knowledge, the first quantitative detection of nidA in PAH-degrading communities, this also presents the first demonstration of a link between nidA abundance to function (pyrene mineralization) in indigenous microbial communities. The observed relationship is consistent with work by Wang et al. (34), who found a positive correlation between pyrene mineralization and abundance of inoculated Mycobacterium sp. PYR-1 in soil slurries. Our observations suggest that nagAc and nidA may be valuable as molecular biomarkers for high molecular weight PAH degradation; especially of interest in historically contaminated systems similar to Chattanooga Creek.

FIGURE 5. Neighbor-joining tree of 16S rDNA sequences from Chattanooga Creek retrieved using Mycobacterium-specific primers. Included are published 16S rDNA sequences from other fast-growing PAH-degrading Mycobacterium (2), fast-growing non-PAH-degrading M. mucogenicum (9), slow-growing M. ulcerans (b), and Pseudomonas putida as an outgroup (O). Boostrap values (for 5000 iterations) over 50% are indicated on branches. indicating that Mycobacterium and Ralstonia sp. U2-like bacteria coexist and are likely responding to similar ecological cues (such as contaminant concentrations and bioavailability). Enrichment and coexistence of genotypes for low- and high-molecular weight PAH degradation are consistent with the complex contaminant mixture in Chattanooga Creek. In contrast to the relationship between nagAc and nidA, no strong relationship was observed between nahAc and either of the other catabolic genes or mineralization rates; however, there was a significant positive correlation between nahAc and naphthalene concentrations. Sanseverino et al. (19) found a similar positive relationship between nahAc abundance and naphthalene concentrations above a threshold of about 100 mg/kg soil, postulating that at low concentrations of naphthalene, organisms with nahAc may

The presence and enrichment of fast-growing Mycobacterium in Chattanooga Creek sediments has been established through quantification of pyrene dioxygenase genes. As all nidA-carrying organisms identified thus far have been fastgrowing Mycobacteria, sequencing of a fast-growing Mycobacterium 16S rDNA clone library was done to examine the phylogenetic distribution of the population with these functional genes. Most clones sequenced were unique, revealing a high diversity of fast-growing Mycobacterium that is only partially represented by our clone library. The majority of sequences clustered with other cultured PAH-degrading species: this observation, combined with detection of nidA genes in this community, suggests that many of these uncultured Mycobacteria could also be capable of high molecular weight PAH degradation. Other studies have also found a high diversity of Mycobacteria in PAH-contaminated soils (16, 17). Interestingly, two sequences were most similar to M. mucogenicum, a fast-growing opportunistic human pathogen which has been reported to be frequently found in surface waters (35). Finding these sequences is not surprising considering the highly populated area of Chattanooga is nearby. The demonstrated diversity of Mycobacteria, combined with the presence of nidA genes, indicates that PAHdegrading Mycobacterium spp. are present in Chattanooga Creek sediments, and likely playing an important role in attenuation of PAHs.

Acknowledgments This work was supported by the Center for Environmental Biotechnology, Research Center of Excellence, University of Tennessee, and the Waste Management Research and Education Institute at the University of Tennessee. Special thanks to F. Menn, J. Easter, J. Rainey, D. Williams, C. Lalande, and V. Vulava for assistance in field sampling, and to L. McKay, J. Sanseverino and three anonymous reviewers for constructive comments on this manuscript. DNA sequencing was done by Joseph May at the Molecular Biology Resource Facility at the University of Tennessee. VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available Data showing the linearity of the nidA real-time assay over a range of template concentrations (Figure S1); an alternate presentation of catabolic gene abundances (Figure S2); the correlation between catabolic genes (Figure S3); and measured PAH concentrations (Table S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 16, 2007. Revised manuscript received May 8, 2007. Accepted May 16, 2007. ES070406C